Electron microscopical techniques, including cytochemistry and freeze-fracture procedures, will be employed to explore membrane structural changes in: (1) slime molds during the cell cycle, sexual fusion, and during the change from haploid to diploid state; (2) prokaryotes that have an altered membrane lipid composition which results from a mutation which blocks cell division in its final stages. Test organisms will be the slime molds Physarum polycephalum, P. flavicomum, Didymium iridis, and Dictyostelium discoideum; and the bacteria Escherichia coli and Agmenellum quadruplicatum. The effect of a soluble fusion inducing substance from one clone of D. discoideum on membranes of a compatible clone will be investigated, as will changes in the membranes of Didymium iridis myxamoebae when compatible clones are mixed. To clearly visualize the cell coat in order to detect membrane surface changes involved in sexual fusion and recognition phenomena, a new staining procedure which incorporates Alcian blue and Ruthenium red in the fixatives has been developed and will be employed extensively. Additional structural changes in the plasmodial plasmalemma of D. iridis will be studied by characterizing the conformational behavior of a unique tetrad of intramembranous particles under varying physiological conditions. Biochemical studies on the plasmodial membrane will be initiated. Intracellular membranes will be investigated by freeze-fracture of the myxogastrian plasmodium.